MacAddress for topic name - Arduino IDE MQTT

Question

I want to use the mac address of my system as a topic name.

I want something like : project/00:1B:44:11:3A:B7/temperature/status

I tried in this way:

#define TEMP_STATUS_TOPIC "project/" + WiFi.macAddress() + "temperature/status"   
#define TEMP_CONTROL_TOPIC "project/temperature/control"

But I get this error:

no matching function for call to 'MQTTClient::publish(StringSumHelper&, char [128], size_t&)'

Any tip will be greatly appriciated!

EDIT:

I am using mqtt.fx client.

Here is where I call publish:

sensors.requestTemperatures(); 
  float t = sensors.getTempCByIndex(0);
  bool static led_temp_status = HIGH;
  
  const int capacity = JSON_OBJECT_SIZE(3);
  StaticJsonDocument<capacity> poolsystem;
  
  if (t < destemp) {
    led_temp_status = LOW;
    digitalWrite(LED_EXTERNAL_TEMP, led_temp_status);
    const int capacity = JSON_OBJECT_SIZE(3);
StaticJsonDocument<capacity> poolsystem;

    poolsystem["temp"] = t;
    poolsystem["heatstatus"] = "on";
    char buffer[128];
    size_t n = serializeJson(poolsystem, buffer);
    Serial.print(F("JSON message: "));
    Serial.println(buffer);
    mqttClient.publish(TEMP_STATUS_TOPIC, buffer, n);   
  } else {
    led_temp_status = HIGH;
    digitalWrite(LED_EXTERNAL_TEMP, led_temp_status);

    poolsystem["temp"] = t;
    poolsystem["heatstatus"] = "off";
    char buffer[128];
    size_t n = serializeJson(poolsystem, buffer);
    Serial.print(F("JSON message: "));
    Serial.println(buffer);
    mqttClient.publish(TEMP_STATUS_TOPIC, buffer, n); 
  }

[Edit] the question to include a [mre]. Where do you call `publish`? Where/how do you use your macros? — 1201ProgramAlarm, May 24 '21 at 21:13
Thank you for your answer! I've edited my question with some more code hoping it helps :) — Radix, May 24 '21 at 21:33
I think is publish(java.lang.String topic, byte[] payload, int qos, boolean retained) — Radix, May 24 '21 at 21:53

score 3 · Answer 1 · answered May 24 '21 at 23:02

You are using a #define but trying to do string additions like its a variable. Keep in mind that the #define primitive is just going to substitute into the code what you put after it. That error is telling you that there is no function call for publish() that has a StringSumHelper& parameter, which is what that #define is putting down.

Use a variable to figure out the MQTT Topic, then use that var in your publish() call.

score 1 · Accepted Answer · answered May 25 '21 at 09:27

First you shouldn't use a #define in that way, like pointed out in another answer.

You could instead declare TEMP_STATUS_TOPIC as a const String:

const String TEMP_STATUS_TOPIC = "project/" + WiFi.macAddress() + "temperature/status";

The problem with MQTTClient::publish() is that the first argument requires a C string, const char*, for the topic name. Replace the lines with:

mqttClient.publish(TEMP_STATUS_TOPIC.c_str(), buffer, n);

MacAddress for topic name - Arduino IDE MQTT

2 Answers2